With the remarkable development of information communication technology and increase of the amount of information in recent years, more precision and more quickness are required for the transmission of signal information. Along with this, the use of high frequency bands is rapidly increasing. In particular, the full-scale use of the frequency band over 1 GHz, especially, a frequency band between 10 to 20 GHz, has been started. As a result, in satellite broadcasting and satellite communication, a method for transmitting and receiving radio waves with Luneberg lens antennas is expected to be developed as an alternative to the conventional method using parabolic antennas.
In the conventional system of satellite broadcasting and satellite communication using parabolic antennas, a geostationary satellite is used in combination with a parabolic antenna oriented in fixed direction to transmit and receive radio waves. With this system, in order to transmit and receive radio waves to and from a plurality of satellites, it is necessary to change the orientation of the antenna depending on the location of the target satellite or to use a plurality of parabolic antennas. On the contrary, a Luneberg lens antenna (a spherical or hemispherical antenna provided with a Luneberg dielectric lens) can transmit and receive radio waves to and from a plurality of stationary satellites when a plurality of feeds are located on the focal position of the Luneberg lens on a cover of the antenna. Also, when a satellite or antenna as a target of communication moves as in the case of a low earth orbit satellite (LEO), the entire antenna should track the target in the case of a parabolic antenna whereas, in the case of a Luneberg lens antenna, only a small component thereof such as a receiver or transmitter should track the target. Thus, a Luneberg lens antenna does not require a large driving system and is also suitable as an antenna for a mobile body. According to the method using a Luneberg lens antenna, a large amount of information can be transmitted and received with one antenna in each residence. That is, a Luneberg lens antenna is also suitable as an antenna for receiving TV broadcasts in the age of multi-channel broadcasting.
A Luneberg lens antenna is provided with a Luneberg dielectric lens having a function of converging and focusing radio waves. The material for the Luneberg dielectric lens must have excellent dielectric characteristics (such as a uniform dielectric constant and a low dielectric loss tangent) to deal with an increasing amount of information, that is, high frequency radio waves. Also, since the antenna is usually installed on the roof of each residence, the material should be small in size and light in weight in view of efficiency and safety of the installation work.
A Luneberg dielectric lens has a spherical or hemispherical shape and comprises a plurality of concentrically stacked layers having different dielectric constants such that the dielectric constant varies, theoretically from 2 to 1, with the innermost center layer having a dielectric constant of about 2 and the outermost layer having a dielectric constant of about 1. Thus, theoretically, a Luneberg dielectric lens is so designed that the dielectric constant ∈r varies from the center (r=0) to the surface (r=R) according to the equation (1) below:∈r=2−(r/R)2   (1)wherein ∈r, R and r represent the dielectric constant, the radius of the lens, and the radius at the measuring point, respectively. The dielectric constant of each of the layers is determined with reference to the value determined by the above equation (1).
In reality, however, since a molded product in which the dielectric constant is continuously varied according to an ideal curve given by the equation (1) is difficult to obtain, a Luneberg dielectric lens is produced by combining a plurality of discrete layers having different dielectric constants. One dielectric lens of a Luneberg-type is disclosed in U. S. Patent Published Application No. 20040029985. The dielectric lens is in the form of a sphere having a core and a multiplicity of hollow spherical shells having different dielectric constants, the spherical shells surrounding the core and being concentrically overlapped to each other to form a concentric sphere. The core and the shells are each made of a foam of a synthetic resin containing a dielectric inorganic filler. Since the dielectric lens is light in weight, it can ensure the workability and safety of the installation work. However, the antenna using the dielectric lens disclosed in U. S. Patent Published Application No. 20040029985 is not enough to satisfy performance, such as antenna gain, for practical use.